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We are conducting a study to determine the ELF (epithelial lining fluid) penetration of our drug. The study design is as follows. Twenty-five volunteers will be given our drug by an infusion. Five subjects per time-point (1, 2, 4, 6 and 8 hours) will have ELF fluid samples taken along with the corresponding plasma sample. The ELF to plasma ratios will then be determined.
My question is what method should be used to determine these ratios. Should we take the concentration in ELF/concentration in plasma at every time-point or should we look at the AUC of ELF/AUC Plasma to come up with one ratio? I am tempted to take the AUC ratio approach because it will take issues such as equilibration lag-time into account. It may also help normalize the variability to a great extent. However, due to the nature of the calculation, all we will have is one number without any estimate of variability to it. On the other hand, calculating the ratio at every time point helps us get more numbers and therefore a better understanding of the variability. However, as can be expected, the ratios at later time-points may be more erroneous.
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Point to point ELF/plasma conc. ratio will give you more clear picture. However once you got the data it is also easy to calculate the AUC(ELF)/AUC(plasma) ratio as well.
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